Editor's note: The following article has been revised from its original version.

A new, potentially fatal syndrome in horses called equine fescue edema has been reported by Australian scientists in the Australian Veterinary Journal. The researchers noted that all affected horses in the study were grazing pastures sown with Mediterranean (winter active) varieties of tall fescue containing endophytes, and have suggested that a combination of fescue variety and endophyte strain might be the culprit.

Most tall fescue planted in the United States is of different, Continental (summer active), varieties (such as KY-31). There have been no reported cases of equine fescue edema in horses grazing Continental tall fescue varieties with this endophyte strain.

"This is a new form of toxicity to horses in any country; it has never been encountered anywhere before," explained Christopher A. Bourke, PhD, a principal research scientist at the Orange Agricultural Institute in New South Wales, Australia, and the lead researcher on the study.

Between October 2007 and December 2008, 48 of 56 horses on six farms in different Australian states rapidly came down with signs of what is being called equine fescue edema after grazing certain grasses under certain environmental conditions (extended drought followed by significant rainfall). Their clinical signs included loss of appetite, depression, and swelling of the head, neck, chest, and abdomen. Four of the horses died, some others recovered quickly and completely, and some mares had longer-term reproductive compromise.

"The edema (fluid swelling) that occurs is the result of falling blood albumin levels [proteins in the blood]," Bourke explained. "It can be seen in the head, neck, brisket, and belly areas, but it will also be occurring in the wall of the large bowel and in the wall of the uterus (in mares). But this internal edema will not be visible to the naked eye."

Also invisible to the naked eye is the symbiotic endophyte fungus within infected tall fescue, which produces substances that act as natural insecticides, thereby making endophyte-infected fescue a hardier grass than uninfected versions. Unfortunately, some of those substances can cause problems in animals that graze the infected fescue (including a compound called ergovaline, which causes a different syndrome termed fescue toxicosis in pregnant mares).

Since endophyte makes the grass tougher, it is desirable from a plant management standpoint. To gain a hardy grass for pastures without the risk of fescue toxicosis, the Mediterranean tall fescue varieties mentioned in the current study were inoculated with a naturally occurring endophyte strain different from the wild-type endophyte often present in infected Continental tall fescue varieties that produces ergovaline. However, the Australian researchers have suggested that this particular endophyte strain, when combined with Mediterranean tall fescue varieties, might produce a different compound that causes equine fescue edema.

"The emergence of this new condition only in association with pastures sown with Mediterranean tall fescue varieties with [this particular] endophyte suggests that a unique interaction is occurring in this specific pairing," wrote the researchers.

Bourke noted that equine fescue edema requires immediate attention by a veterinarian, who should do blood work to measure blood protein levels. "If the albumin level falls too low, the horse may need plasma fluid therapy to keep it alive," he said.

No confirmed cases of equine fescue edema have been reported in the U.S., and several years of trials with this endophyte/Mediterranean-type fescue combination in Mississippi have failed to duplicate the Australian experience, according to Ray Smith, MS, PhD, forage extension specialist at the University of Kentucky.

If it is proven that Mediterranean fescue with this endophyte is indeed the culprit in equine fescue edema, then the only preventive measure will be to remove this fescue variety/endophyte strain combination from horse pastures, or to remove the horses from such pastures. Fortunately, Smith noted that in the U.S., Mediterranean-type tall fescues have only been sown in areas of western Texas and Oklahoma with full awareness of the potential horse issues; these landowners are successfully using this fescue to raise cattle and sheep with no ill effects.

The study, "Fescue-associated oedema of horses grazing on endophyte-inoculated tall fescue grass (Festuca arundinacea) pastures" was published in the December 2009 issue of The Australian Veterinary Journal. The abstract is available online.